The use of agrochemicals is essential in farms for stable food production. However, a large amount of synthesized agrochemicals containing substances not found in nature are applied to land for a long period of time, thus bringing about harm. The most serious problem is their direct influences on human and domestic animals and their destruction of the ecological system. In recent years, the harmful influences on human and domestic animals have been reduced owing to drastic improvements in the specificity of agrochemicals for weeds, harmful insects and disease germs, but one should still pay adequate attention when coming into contact with a high concentration of agrochemicals in applying thereof. Further, the selectivity of agrochemicals for weeds, harmful insects and disease germs is not necessarily high, resulting often in disturbance of the ecological system.
To solve the problem, there is a need for development of pesticides not adversely affecting other species than the target species to protect the environment. One example of such pesticides is herbicides using weed pathogens. Certain weed pathogens are ubiquitous in weeds, and it is known that such pathogens can be used in a herbicide with little or no damage to humans, domestic animals, and small creatures including fishes, insects etc. and with less phytotoxicity on plants. Further, the specificity of such herbicides using pathogens for the target weed is so high that their selectivity is considered high and the ecological system is hardly disturbed. Examples of herbicides using weed pathogens commercially available at present are a herbicide DeVine using Phytophthora palmivora for Stranglervine, a herbicide Collego using Colletotrichum gloeosporioides f. sp. aeschynomene for Northern jointvetch, and a herbicide BioMal using Colletotrichum gloeosporioides f. sp. malvae for round-leaved mallow.
Examples of herbicides using weed pathogens for controlling Echinochloa spp. as the most serious weed in paddy fields are Cochliobolus lunatus (Anamorph: Curvularia lunata) [Weed Research, 27, 43-47, (1987) and Japanese Patent Application Laid-Open Publication No. 284,963/93], Ustilago trichophora [w093/05656], and Drechslera monoceras (synonym of Exserohilum monoceras) [Japanese Patent Application Laid-Open Publication Nos. 219,883/91, 226,905/92, 360,678/92, 370,090/92, 277,042/94, 329,513/94, and 247,822/94].
However, these conventional herbicides are disadvantageous:
for example, Cochliobolus lunatus needs dew period for more than 18 hours to demonstrate its sufficient effect; Ustilago trichophora needs 4 to 5 weeks after spraying until its sufficient effect appears; and Drechslera monoceras produces a small amount of spores. Therefore, herbicides using these fungi still have not come into practical use.